1. Field of the Invention
The present invention relates to a method and apparatus for handling packet discard in a wireless communications system, and more particularly, to a method and apparatus for preventing stall of a receiving window and enhancing transmission efficiency in the wireless communications system.
2. Description of the Prior Art
The third generation (3G) mobile telecommunications system has adopted a Wideband Code Division Multiple Access (WCDMA) wireless air interface access method for a cellular network. WCDMA provides high frequency spectrum utilization, universal coverage, and high quality, high-speed multimedia data transmission. The WCDMA method also meets all kinds of QoS requirements simultaneously, providing diverse, flexible, two-way transmission services and better communication quality to reduce transmission interruption rates.
The access stratum of the 3G mobile telecommunications system comprises a radio resource control (RRC), radio link control (RLC), media access control (MAC), packet data convergence protocol (PDCP), broadcast/multicast control (BMC) and other sub-layers of different functions. The operations of the above-mentioned sub-layers are well known for those skilled in the art, and will not be further mentioned. A primary function of the RLC layer is providing different transmission quality processing, performing segmentation, reassembly, concatenation, padding, retransmission, sequence check, and duplication detection on transmitted data or control instructions based on different transmission quality requirements. The MAC layer can match packets received from different logic channels of the RLC layer to common, shared, or dedicated transport channels according to radio resource allocation commands of the RRC layer, for performing channel mapping, multiplexing, transport format selection, or random access control.
When the RLC layer operates in an acknowledged mode (AM), a header of an RLC PDU (Protocol Data Unit) comprises a two-bit HE (Header Extension) field utilized for indicating if the next octet is data or a Length Indicator (LI) and Extension (E) bit. The HE field has different values, and the corresponding description is as follows:
1. “00”: The succeeding octet contains data.
2. “01”: The succeeding octet contains LI and E bit.
3. “10” and “11”: Reserved. PDUs with this coding represents that a protocol error is occurred, and the PDUs will be discarded.
In order to decrease overhead, the prior art can trigger a “use of the special value of the HE field” mode to set “10” of the HE field to indicate that the succeeding octet contains data and the last octet of the corresponding PDU is the last octet of an SDU (Service Data Unit). In other words, if an SDU ends at the end of a PDU, the HE field of the PDU will be set to “10.” As a result, an extra PDU carrying the corresponding LI is not needed, to decrease overhead.
In some situations, such as transmission timeout or over maximum allowed times of retransmission, the transmitter can trigger an SDU discard with explicit signalling procedure, and uses an MRW SUFI (Move Receiving Window Super Field) in a STATUS PDU to request the receiver to move its receiving window to discard specified SDUs. After the SDU discard with explicit signalling procedure is triggered, the transmitter discards all SDUs to be discarded, and discards PDUs including segments of the discarded SDUs or LIs indicating the end of the SDUs. Then, the transmitter sends the MRW SUFI to inform the receiver about discarding the SDUs, so as to complete the SDU discard with explicit signalling procedure.
As mentioned above, in the “use of the special value of the HE field” mode, if an SDU ends at the end of a PDU, the extra PDU carrying the corresponding LI is not needed. In such a situation, the method of setting the contents of MRW SUFI needs to be modified, so as to support the “use of the special value of the HE field” mode.
Detail description of the MRW SUFI can be found in the related protocol specification. The MRW SUFI comprises fields of:
1. LENGTH: The number of discarded SDU.
2. SN_MRWi: Each SN_MRW is used to indicate the end of each discarded SDU. The last SN_MRW, i.e. SN_MRWLENGTH requests the receiver to discard all not yet successfully received SDUs that have segments or LIs indicating the end of the SDUs in the PDUs with Sequence Numbers <SN_MRWLENGTH.
3. NLENGTH: NLENGTH is used together with SN_MRWLENGTH to indicate the end of the last SDU to be discarded in the receiver.
To support the “use of the special value of the HE field” mode, the prior art provides operations for setting the MRW SUFI as follows. If the “use of the special value of the HE field” mode is triggered, and the last SDU to be discarded ends at the end of a PDU, i.e. the PDU does not contain an LI corresponding to the last SDU to be discarded, then the prior art sets the SN_MRWLENGTH to be a sequence number (SN) “a” of the PDU, and sets NLENGTH to be “0000.” Thus, the receiver will advance its receiving window to start from SN=a. However, as mentioned above, after the SDU discard with explicit signalling procedure is triggered, the transmitter discards all SDUs to be discarded, and discards PDUs including segments of the discarded SDUs or LIs indicating the end of the SDUs. In other words, PDU with SN=a has been discarded by the transmitter in this example, and will not be transmitted to the receiver. In such a situation, the receiver waits for the PDU with SN=a, so that the receiving window is stalled until another MRW procedure is triggered. The transmission efficiency and transport data rate are reduced.
In short, when the “use of the special value of the HE field” mode is triggered, and the last SDU to be discarded ends at the end of a PDU, the prior art sets SN_MRWLENGTH to SN of the PDU, to advance the receiving window of the receiver to start from the SN of the PDU. However, since the PDU has been discarded by the transmitter, the receiver cannot receive the PDU anymore, causing stall of the receiving window and degradation of transmission efficiency.